1. Field of the Invention
The present invention relates to a delay line for traveling wave tubes, in particular for the amplification of mm waves, in which a wave guide is provided with outward-leading transverse walls, each of which have a central electron beam opening and at least one coupling opening and which form a line cell with the adjoining transverse wall, and which contains two portions having different constant delays for the line wave, where the delay of the front line portion, as viewed in the direction of the electron beam, is less than the delay of the rear line portion, and more particularly to such an arrangement wherein there is provided a transition section arranged between the two portions.
2. Description of the Prior Art
A delay line of the general type set forth above is described, for example, in the publication "Microwave System News," June/July 1974, Pages 64C--64F.
Inasmuch as an optimum transition of energy between the electron beam and the accompanying line wave is dependent upon a specific difference between the beam speed and the wave speed ("synchronism condition"), and as, furthermore, the electron beam which constantly emits energy becomes increasingly slower toward the end of the line, as is known a high tube efficiency is achieved if the delay of the line is increased toward its output in accordance with the speed loss of the electron beam. A measure of this type, which is also referred to as "resynchronization by speed tapering" is described in detail, for example, in the general articles by M. K. Scherba, in the publication "Microwaves", July 1973, Pages 46-51, and J. T. Mendel in the "Proceedings of the IEEE", Vol. 61, No. 3, March 1973, Pages 280-298, in particular at Page 296, left-hand column, paragraph 3 to Page 297 right-hand column, paragraph 1.
In the type of delay line referred to in the introduction, the speed matching is achieved in accordance with a principle explained by reference to a coil in FIG. 2a of the above-quoted publication "Microwaves": two line portions having differing, constant phase speed connected to one another over an intermediate portion which comprises a plurality of line periods, and which constantly converts the phase speeds one to another. In a coupled-cavity line, this intermediate portion must consist of a number of differently designed line cells, which lead to incompatibilities; the differing geometric patterns of the cells complicate their production. Furthermore, even with the most accurate calculations, and the most careful operation, the cell sequence can only completely fulfill the requirements to which it is subject (speed matching with a surge impedance transformation which is as low-reflective as possible, and maintaining a high degree of amplification), as the ideal, constant transitions must be approximated by means of stepped functions. In particular, correction of production-related mismatchings, for example due to dimensioning variations of individual line discs, prove extremely expensive and complicated. This is one of the reasons why recently the "voltage jump" technique has been recommended in place of the speed tapering technique, as being more practical for lines of the coupled-cavity type, in particular in the mm wave range, although the subsequent acceleration of the electron beam produces insulation problems in this case. In this connection, one may take reference to the two aforementioned articles of Scherba and Mendel.